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 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package com.hisense.davidmoten.rtree.internal.util;

import java.util.AbstractQueue;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.SortedSet;

/**
 * An unbounded priority {@linkplain Queue queue} based on a priority heap. The
 * elements of the priority queue are ordered according to their
 * {@linkplain Comparable natural ordering}, or by a {@link Comparator} provided
 * at queue construction time, depending on which constructor is used. A
 * priority queue does not permit {@code null} elements. A priority queue
 * relying on natural ordering also does not permit insertion of non-comparable
 * objects (doing so may result in {@code ClassCastException}).
 *
 * <p>
 * The <em>head</em> of this queue is the <em>least</em> element with respect to
 * the specified ordering. If multiple elements are tied for least value, the
 * head is one of those elements -- ties are broken arbitrarily. The queue
 * retrieval operations {@code poll}, {@code remove}, {@code peek}, and
 * {@code element} access the element at the head of the queue.
 *
 * <p>
 * A priority queue is unbounded, but has an internal <i>capacity</i> governing
 * the size of an array used to store the elements on the queue. It is always at
 * least as large as the queue size. As elements are added to a priority queue,
 * its capacity grows automatically. The details of the growth policy are not
 * specified.
 *
 * <p>
 * This class and its iterator implement all of the <em>optional</em> methods of
 * the {@link Collection} and {@link Iterator} interfaces. The Iterator provided
 * in method {@link #iterator()} is <em>not</em> guaranteed to traverse the
 * elements of the priority queue in any particular order. If you need ordered
 * traversal, consider using {@code Arrays.sort(pq.toArray())}.
 *
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> Multiple
 * threads should not access a {@code PriorityQueue} instance concurrently if
 * any of the threads modifies the queue. Instead, use the thread-safe
 * {@link java.util.concurrent.PriorityBlockingQueue} class.
 *
 * <p>
 * Implementation note: this implementation provides O(log(n)) time for the
 * enqueuing and dequeuing methods ({@code offer}, {@code poll},
 * {@code remove()} and {@code add}); linear time for the {@code remove(Object)}
 * and {@code contains(Object)} methods; and constant time for the retrieval
 * methods ({@code peek}, {@code element}, and {@code size}).
 *
 * <p>
 * This class is a member of the <a href=
 * "{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
 * Framework</a>.
 *
 * @since 1.5
 * @author Josh Bloch, Doug Lea
 * @param <E>
 *            the type of elements held in this collection
 */
public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable {

    private static final long serialVersionUID = -7720805057305804111L;

    private static final int DEFAULT_INITIAL_CAPACITY = 11;

    /**
     * Priority queue represented as a balanced binary heap: the two children of
     * queue[n] are queue[2*n+1] and queue[2*(n+1)]. The priority queue is
     * ordered by comparator, or by the elements' natural ordering, if
     * comparator is null: For each node n in the heap and each descendant d of
     * n, n <= d. The element with the lowest value is in queue[0], assuming the
     * queue is nonempty.
     */
    transient Object[] queue; // non-private to simplify nested class access

    /**
     * The number of elements in the priority queue.
     */
    private int size = 0;

    /**
     * The comparator, or null if priority queue uses elements' natural
     * ordering.
     */
    private final Comparator<? super E> comparator;

    /**
     * The number of times this priority queue has been <i>structurally
     * modified</i>. See AbstractList for gory details.
     */
    transient int modCount = 0; // non-private to simplify nested class access

    /**
     * Creates a {@code PriorityQueue} with the default initial capacity (11)
     * that orders its elements according to their {@linkplain Comparable
     * natural ordering}.
     */
    public PriorityQueue() {
        this(DEFAULT_INITIAL_CAPACITY, null);
    }

    /**
     * Creates a {@code PriorityQueue} with the specified initial capacity that
     * orders its elements according to their {@linkplain Comparable natural
     * ordering}.
     *
     * @param initialCapacity
     *            the initial capacity for this priority queue
     * @throws IllegalArgumentException
     *             if {@code initialCapacity} is less than 1
     */
    public PriorityQueue(int initialCapacity) {
        this(initialCapacity, null);
    }

    /**
     * Creates a {@code PriorityQueue} with the default initial capacity and
     * whose elements are ordered according to the specified comparator.
     *
     * @param comparator
     *            the comparator that will be used to order this priority queue.
     *            If {@code null}, the {@linkplain Comparable natural ordering}
     *            of the elements will be used.
     * @since 1.8
     */
    public PriorityQueue(Comparator<? super E> comparator) {
        this(DEFAULT_INITIAL_CAPACITY, comparator);
    }

    /**
     * Creates a {@code PriorityQueue} with the specified initial capacity that
     * orders its elements according to the specified comparator.
     *
     * @param initialCapacity
     *            the initial capacity for this priority queue
     * @param comparator
     *            the comparator that will be used to order this priority queue.
     *            If {@code null}, the {@linkplain Comparable natural ordering}
     *            of the elements will be used.
     * @throws IllegalArgumentException
     *             if {@code initialCapacity} is less than 1
     */
    public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) {
        // Note: This restriction of at least one is not actually needed,
        // but continues for 1.5 compatibility
        if (initialCapacity < 1)
            throw new IllegalArgumentException();
        this.queue = new Object[initialCapacity];
        this.comparator = comparator;
    }

    /**
     * Creates a {@code PriorityQueue} containing the elements in the specified
     * collection. If the specified collection is an instance of a
     * {@link SortedSet} or is another {@code PriorityQueue}, this priority
     * queue will be ordered according to the same ordering. Otherwise, this
     * priority queue will be ordered according to the {@linkplain Comparable
     * natural ordering} of its elements.
     *
     * @param c
     *            the collection whose elements are to be placed into this
     *            priority queue
     * @throws ClassCastException
     *             if elements of the specified collection cannot be compared to
     *             one another according to the priority queue's ordering
     * @throws NullPointerException
     *             if the specified collection or any of its elements are null
     */
    @SuppressWarnings("unchecked")
    public PriorityQueue(Collection<? extends E> c) {
        if (c instanceof SortedSet<?>) {
            SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
            this.comparator = (Comparator<? super E>) ss.comparator();
            initElementsFromCollection(ss);
        } else if (c instanceof PriorityQueue<?>) {
            PriorityQueue<? extends E> pq = (PriorityQueue<? extends E>) c;
            this.comparator = (Comparator<? super E>) pq.comparator();
            initFromPriorityQueue(pq);
        } else {
            this.comparator = null;
            initFromCollection(c);
        }
    }

    /**
     * Creates a {@code PriorityQueue} containing the elements in the specified
     * priority queue. This priority queue will be ordered according to the same
     * ordering as the given priority queue.
     *
     * @param c
     *            the priority queue whose elements are to be placed into this
     *            priority queue
     * @throws ClassCastException
     *             if elements of {@code c} cannot be compared to one another
     *             according to {@code c}'s ordering
     * @throws NullPointerException
     *             if the specified priority queue or any of its elements are
     *             null
     */
    @SuppressWarnings("unchecked")
    public PriorityQueue(PriorityQueue<? extends E> c) {
        this.comparator = (Comparator<? super E>) c.comparator();
        initFromPriorityQueue(c);
    }

    /**
     * Creates a {@code PriorityQueue} containing the elements in the specified
     * sorted set. This priority queue will be ordered according to the same
     * ordering as the given sorted set.
     *
     * @param c
     *            the sorted set whose elements are to be placed into this
     *            priority queue
     * @throws ClassCastException
     *             if elements of the specified sorted set cannot be compared to
     *             one another according to the sorted set's ordering
     * @throws NullPointerException
     *             if the specified sorted set or any of its elements are null
     */
    @SuppressWarnings("unchecked")
    public PriorityQueue(SortedSet<? extends E> c) {
        this.comparator = (Comparator<? super E>) c.comparator();
        initElementsFromCollection(c);
    }

    private void initFromPriorityQueue(PriorityQueue<? extends E> c) {
        if (c.getClass() == PriorityQueue.class) {
            this.queue = c.toArray();
            this.size = c.size();
        } else {
            initFromCollection(c);
        }
    }

    private void initElementsFromCollection(Collection<? extends E> c) {
        Object[] a = c.toArray();
        // If c.toArray incorrectly doesn't return Object[], copy it.
        if (a.getClass() != Object[].class)
            a = Arrays.copyOf(a, a.length, Object[].class);
        int len = a.length;
        if (len == 1 || this.comparator != null)
            for (int i = 0; i < len; i++)
                if (a[i] == null)
                    throw new NullPointerException();
        this.queue = a;
        this.size = a.length;
    }

    /**
     * Initializes queue array with elements from the given Collection.
     *
     * @param c
     *            the collection
     */
    private void initFromCollection(Collection<? extends E> c) {
        initElementsFromCollection(c);
        heapify();
    }

    /**
     * The maximum size of array to allocate. Some VMs reserve some header words
     * in an array. Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * Increases the capacity of the array.
     *
     * @param minCapacity
     *            the desired minimum capacity
     */
    private void grow(int minCapacity) {
        int oldCapacity = queue.length;
        // Double size if small; else grow by 50%
        int newCapacity = oldCapacity
                + ((oldCapacity < 64) ? (oldCapacity + 2) : (oldCapacity >> 1));
        // overflow-conscious code
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        queue = Arrays.copyOf(queue, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws ClassCastException
     *             if the specified element cannot be compared with elements
     *             currently in this priority queue according to the priority
     *             queue's ordering
     * @throws NullPointerException
     *             if the specified element is null
     */
    @Override
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @return {@code true} (as specified by {@link Queue#offer})
     * @throws ClassCastException
     *             if the specified element cannot be compared with elements
     *             currently in this priority queue according to the priority
     *             queue's ordering
     * @throws NullPointerException
     *             if the specified element is null
     */
    @Override
    public boolean offer(E e) {
        if (e == null)
            throw new NullPointerException();
        modCount++;
        int i = size;
        if (i >= queue.length)
            grow(i + 1);
        size = i + 1;
        if (i == 0)
            queue[0] = e;
        else
            siftUp(i, e);
        return true;
    }

    @Override
    @SuppressWarnings("unchecked")
    public E peek() {
        return (size == 0) ? null : (E) queue[0];
    }

    private int indexOf(Object o) {
        if (o != null) {
            for (int i = 0; i < size; i++)
                if (o.equals(queue[i]))
                    return i;
        }
        return -1;
    }

    /**
     * Removes a single instance of the specified element from this queue, if it
     * is present. More formally, removes an element {@code e} such that
     * {@code o.equals(e)}, if this queue contains one or more such elements.
     * Returns {@code true} if and only if this queue contained the specified
     * element (or equivalently, if this queue changed as a result of the call).
     *
     * @param o
     *            element to be removed from this queue, if present
     * @return {@code true} if this queue changed as a result of the call
     */
    @Override
    public boolean remove(Object o) {
        int i = indexOf(o);
        if (i == -1)
            return false;
        else {
            removeAt(i);
            return true;
        }
    }

    /**
     * Version of remove using reference equality, not equals. Needed by
     * iterator.remove.
     *
     * @param o
     *            element to be removed from this queue, if present
     * @return {@code true} if removed
     */
    boolean removeEq(Object o) {
        for (int i = 0; i < size; i++) {
            if (o == queue[i]) {
                removeAt(i);
                return true;
            }
        }
        return false;
    }

    /**
     * Returns {@code true} if this queue contains the specified element. More
     * formally, returns {@code true} if and only if this queue contains at
     * least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o
     *            object to be checked for containment in this queue
     * @return {@code true} if this queue contains the specified element
     */
    @Override
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /**
     * Returns an array containing all of the elements in this queue. The
     * elements are in no particular order.
     *
     * <p>
     * The returned array will be "safe" in that no references to it are
     * maintained by this queue. (In other words, this method must allocate a
     * new array). The caller is thus free to modify the returned array.
     *
     * <p>
     * This method acts as bridge between array-based and collection-based APIs.
     *
     * @return an array containing all of the elements in this queue
     */
    @Override
    public Object[] toArray() {
        return Arrays.copyOf(queue, size);
    }

    /**
     * Returns an array containing all of the elements in this queue; the
     * runtime type of the returned array is that of the specified array. The
     * returned array elements are in no particular order. If the queue fits in
     * the specified array, it is returned therein. Otherwise, a new array is
     * allocated with the runtime type of the specified array and the size of
     * this queue.
     *
     * <p>
     * If the queue fits in the specified array with room to spare (i.e., the
     * array has more elements than the queue), the element in the array
     * immediately following the end of the collection is set to {@code null}.
     *
     * <p>
     * Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs. Further, this method allows
     * precise control over the runtime type of the output array, and may, under
     * certain circumstances, be used to save allocation costs.
     *
     * <p>
     * Suppose {@code x} is a queue known to contain only strings. The following
     * code can be used to dump the queue into a newly allocated array of
     * {@code String}:
     *
     * <pre>
     * {
     *     &#64;code
     *     String[] y = x.toArray(new String[0]);
     * }
     * </pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a
     *            the array into which the elements of the queue are to be
     *            stored, if it is big enough; otherwise, a new array of the
     *            same runtime type is allocated for this purpose.
     * @return an array containing all of the elements in this queue
     * @throws ArrayStoreException
     *             if the runtime type of the specified array is not a supertype
     *             of the runtime type of every element in this queue
     * @throws NullPointerException
     *             if the specified array is null
     */
    @Override
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        final int size = this.size;
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(queue, size, a.getClass());
        System.arraycopy(queue, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    /**
     * Returns an iterator over the elements in this queue. The iterator does
     * not return the elements in any particular order.
     *
     * @return an iterator over the elements in this queue
     */
    @Override
    public Iterator<E> iterator() {
        return new Itr();
    }

    private final class Itr implements Iterator<E> {
        /**
         * Index (into queue array) of element to be returned by subsequent call
         * to next.
         */
        private int cursor = 0;

        /**
         * Index of element returned by most recent call to next, unless that
         * element came from the forgetMeNot list. Set to -1 if element is
         * deleted by a call to remove.
         */
        private int lastRet = -1;

        /**
         * A queue of elements that were moved from the unvisited portion of the
         * heap into the visited portion as a result of "unlucky" element
         * removals during the iteration. (Unlucky element removals are those
         * that require a siftup instead of a siftdown.) We must visit all of
         * the elements in this list to complete the iteration. We do this after
         * we've completed the "normal" iteration.
         *
         * We expect that most iterations, even those involving removals, will
         * not need to store elements in this field.
         */
        private ArrayDeque<E> forgetMeNot = null;

        /**
         * Element returned by the most recent call to next iff that element was
         * drawn from the forgetMeNot list.
         */
        private E lastRetElt = null;

        /**
         * The modCount value that the iterator believes that the backing Queue
         * should have. If this expectation is violated, the iterator has
         * detected concurrent modification.
         */
        private int expectedModCount = modCount;

        @Override
        public boolean hasNext() {
            return cursor < size || (forgetMeNot != null && !forgetMeNot.isEmpty());
        }

        @Override
        @SuppressWarnings("unchecked")
        public E next() {
            if (expectedModCount != modCount)
                throw new ConcurrentModificationException();
            if (cursor < size)
                return (E) queue[lastRet = cursor++];
            if (forgetMeNot != null) {
                lastRet = -1;
                lastRetElt = forgetMeNot.poll();
                if (lastRetElt != null)
                    return lastRetElt;
            }
            throw new NoSuchElementException();
        }

        @Override
        public void remove() {
            if (expectedModCount != modCount)
                throw new ConcurrentModificationException();
            if (lastRet != -1) {
                E moved = PriorityQueue.this.removeAt(lastRet);
                lastRet = -1;
                if (moved == null)
                    cursor--;
                else {
                    if (forgetMeNot == null)
                        forgetMeNot = new ArrayDeque<E>();
                    forgetMeNot.add(moved);
                }
            } else if (lastRetElt != null) {
                PriorityQueue.this.removeEq(lastRetElt);
                lastRetElt = null;
            } else {
                throw new IllegalStateException();
            }
            expectedModCount = modCount;
        }
    }

    @Override
    public int size() {
        return size;
    }

    /**
     * Removes all of the elements from this priority queue. The queue will be
     * empty after this call returns.
     */
    @Override
    public void clear() {
        modCount++;
        for (int i = 0; i < size; i++)
            queue[i] = null;
        size = 0;
    }

    @Override
    @SuppressWarnings("unchecked")
    public E poll() {
        if (size == 0)
            return null;
        int s = --size;
        modCount++;
        E result = (E) queue[0];
        E x = (E) queue[s];
        queue[s] = null;
        if (s != 0)
            siftDown(0, x);
        return result;
    }

    /**
     * Removes the ith element from queue.
     *
     * Normally this method leaves the elements at up to i-1, inclusive,
     * untouched. Under these circumstances, it returns null. Occasionally, in
     * order to maintain the heap invariant, it must swap a later element of the
     * list with one earlier than i. Under these circumstances, this method
     * returns the element that was previously at the end of the list and is now
     * at some position before i. This fact is used by iterator.remove so as to
     * avoid missing traversing elements.
     */
    @SuppressWarnings("unchecked")
    private E removeAt(int i) {
        // assert i >= 0 && i < size;
        modCount++;
        int s = --size;
        if (s == i) // removed last element
            queue[i] = null;
        else {
            E moved = (E) queue[s];
            queue[s] = null;
            siftDown(i, moved);
            if (queue[i] == moved) {
                siftUp(i, moved);
                if (queue[i] != moved)
                    return moved;
            }
        }
        return null;
    }

    /**
     * Inserts item x at position k, maintaining heap invariant by promoting x
     * up the tree until it is greater than or equal to its parent, or is the
     * root.
     *
     * To simplify and speed up coercions and comparisons. the Comparable and
     * Comparator versions are separated into different methods that are
     * otherwise identical. (Similarly for siftDown.)
     *
     * @param k
     *            the position to fill
     * @param x
     *            the item to insert
     */
    private void siftUp(int k, E x) {
        if (comparator != null)
            siftUpUsingComparator(k, x);
        else
            siftUpComparable(k, x);
    }

    @SuppressWarnings("unchecked")
    private void siftUpComparable(int k, E x) {
        Comparable<? super E> key = (Comparable<? super E>) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (key.compareTo((E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = key;
    }

    @SuppressWarnings("unchecked")
    private void siftUpUsingComparator(int k, E x) {
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = queue[parent];
            if (comparator.compare(x, (E) e) >= 0)
                break;
            queue[k] = e;
            k = parent;
        }
        queue[k] = x;
    }

    /**
     * Inserts item x at position k, maintaining heap invariant by demoting x
     * down the tree repeatedly until it is less than or equal to its children
     * or is a leaf.
     *
     * @param k
     *            the position to fill
     * @param x
     *            the item to insert
     */
    private void siftDown(int k, E x) {
        if (comparator != null)
            siftDownUsingComparator(k, x);
        else
            siftDownComparable(k, x);
    }

    @SuppressWarnings("unchecked")
    private void siftDownComparable(int k, E x) {
        Comparable<? super E> key = (Comparable<? super E>) x;
        int half = size >>> 1; // loop while a non-leaf
        while (k < half) {
            int child = (k << 1) + 1; // assume left child is least
            Object c = queue[child];
            int right = child + 1;
            if (right < size && ((Comparable<? super E>) c).compareTo((E) queue[right]) > 0)
                c = queue[child = right];
            if (key.compareTo((E) c) <= 0)
                break;
            queue[k] = c;
            k = child;
        }
        queue[k] = key;
    }

    @SuppressWarnings("unchecked")
    private void siftDownUsingComparator(int k, E x) {
        int half = size >>> 1;
        while (k < half) {
            int child = (k << 1) + 1;
            Object c = queue[child];
            int right = child + 1;
            if (right < size && comparator.compare((E) c, (E) queue[right]) > 0)
                c = queue[child = right];
            if (comparator.compare(x, (E) c) <= 0)
                break;
            queue[k] = c;
            k = child;
        }
        queue[k] = x;
    }

    /**
     * Establishes the heap invariant (described above) in the entire tree,
     * assuming nothing about the order of the elements prior to the call.
     */
    @SuppressWarnings("unchecked")
    private void heapify() {
        for (int i = (size >>> 1) - 1; i >= 0; i--)
            siftDown(i, (E) queue[i]);
    }

    /**
     * Returns the comparator used to order the elements in this queue, or
     * {@code null} if this queue is sorted according to the
     * {@linkplain Comparable natural ordering} of its elements.
     *
     * @return the comparator used to order this queue, or {@code null} if this
     *         queue is sorted according to the natural ordering of its elements
     */
    public Comparator<? super E> comparator() {
        return comparator;
    }

    /**
     * Saves this queue to a stream (that is, serializes it).
     *
     * @serialData The length of the array backing the instance is emitted
     *             (int), followed by all of its elements (each an
     *             {@code Object}) in the proper order.
     * @param s
     *            the stream
     * @throws java.io.IOException
     *             when a problem occurs writing the object
     */
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
        // Write out element count, and any hidden stuff
        s.defaultWriteObject();

        // Write out array length, for compatibility with 1.5 version
        s.writeInt(Math.max(2, size + 1));

        // Write out all elements in the "proper order".
        for (int i = 0; i < size; i++)
            s.writeObject(queue[i]);
    }

    /**
     * Reconstitutes the {@code PriorityQueue} instance from a stream (that is,
     * deserializes it).
     *
     * @param s
     *            the stream
     * @throws java.io.IOException
     *             when a problem occurs reading the object
     * @throws ClassNotFoundException
     *             when an attempt to read the object was made but the class
     *             corresponding to the serialized object was not on the
     *             classpath
     */
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in (and discard) array length
        s.readInt();

        queue = new Object[size];

        // Read in all elements.
        for (int i = 0; i < size; i++)
            queue[i] = s.readObject();

        // Elements are guaranteed to be in "proper order", but the
        // spec has never explained what that might be.
        heapify();
    }

}
